New Details of Ferroelectric Switching

All of our current information technology relies on devices that
process information as binary ones and zeroes. Ferroelectric
materials are of special interest to developers of the next generation
of such devices because they exhibit polarized electronic states
that can represent bits of information. Moreover, these materials
retain their polarization states without consuming electrical
power, making ferroelectrics the subject of intense study for
nonvolatile memory applications in which data is stored even
when the power is turned off. One problem, however, is polarization
fatigue: after a number of cycles, the switchable polarization
begins to taper off, rendering the device unusable. Recently,
a team of researchers from the University of Wisconsin, Bell
Laboratories, and the University of Michigan used the x-ray synchrotron
at the APS to study the micron-scale details of polarization
fatigue in ferroelectric oxides.

"X-rays penetrate right through materials, so we can look deep inside
electronic devices," says Eric Isaacs, former Bell Labs researcher,
now director of Argonne's Center for Nanoscale Materials and a collaborator
in the research. "Here we are studying materials in a realistic structure,
not contrived devices suited to a particular probe."

Polarization fatigue is a well known problem in ferroelectric capacitor
technology. In this case, the effect is believed to stem from migration
of oxygen atoms to the electrode region. This, in turn, leads to
formation of oxygen vacancies which can pin polarization domain walls
and inhibit switching. Another mechanism involves the formation of
a layer near the electrode interface that reduces the total electric
field in the ferroelectric material and shuts down its ability to
reverse polarization. Either way, the proposed mechanisms appear
to be microscopic in nature, and so to study them in detail a precise
high-resolution structural analysis tool is required. X-ray microdiffraction
can image the evolution of polarization domains in buried ferroelectric
thin films during switching with submicrometer resolution.

The ferroelectric devices were made in the University of Wisconsin
group of Chang-Beom Eom by first depositing an SrRuO3 bottom
electrode on an insulating SrTiO3 substrate. Epitaxial
PbZr1-xTixO3 (PZT) films with a
nominal composition x = 0.55 and thicknesses of 80 or 160 nm were
grown on top of the electrodes by radio frequency sputtering, followed
by a top electrode layer consisting of sputtered polycrystalline
platinum. A beam of 10 keV x-rays from the MHATT/XOR facility at
beamline 7-ID of the APS were focused onto a 0.8-m spot on this
ferroelectric structure, and the diffracted x-rays were detected
using conventional x-ray diffraction techniques. Images of the stored
polarization with the ferroelectric layer were obtained by scanning
the x-ray beam across the device. An advantage of this configuration
is that electrical measurements can be made in situ during x-ray
diffraction experiments. As a baseline, diffraction images were collected
for each of the two stable polarization states, which established
that x-ray microdiffraction was an accurate probe of the ferroelectric
behavior.

The results of this study showed that polarization fatigue was qualitatively
different when the switching was driven by lower-amplitude electric-field
pulses (0.625 MV per cm peak) switching versus higher field pulses
(1.2 MV per cm peak). Fatigue was observed in both regimes as the
polarization-field hysteresis loops collapsed after repeated cycling
with triangle wave pulses at 1 kHz. Low electric field fatigue was
observed within 104 pulses as the PZT layer structure
became pinned into an unswitchable state, which could be restored
by exposure to higher field pulses.

A different process was found for fatigue induced by high field
pulses. Although the onset of fatigue occurred after a much higher
number of electric field cycles, the decrease in switchable polarization
and the structural changes were more dramatic and irreversible. The
x-ray microdiffraction images showed that isolated regions of severely
decreased x-ray scattering intensity begin to form and that these
eventually coalesce to encompass the entire region under the electrodes.
The diffraction data indicate that there is a drastic loss of structural
order as the fatigue progresses to failure of the device. "These
are not things you can tell just from an electrical measurement," says
collaborator Paul Evans of the University of Wisconsin. "There seem
to be two sets of mechanisms in the low-field versus the high-field
fatigue, and we can distinguish them by probing the structure with
x-rays."

These measurements, made possible by the high brightness of the
third generation synchrotron at the Advanced Photon Source, confirm
that several mechanisms may be at play during fatigue and failure
of ferroelectric devices. According to Evans, the results also indicate
that x-ray microdiffraction is an ideal tool for high resolution
studies of structural changes in thin film devices under a wide range
of conditions, especially when structural and electronic phenomena
are deeply enmeshed. "Microdiffraction is one of the killer applications
for third generation synchrotrons such as APS." David Voss

This work was supported by the National Science Foundation through
the University of Wisconsin Materials Research Science and Engineering
Center (grant number DMR-0079983) and grant no. DMR- 0313764 (C.B.E.).
E.D. acknowledges support from the U.S. Department of Energy (grant
numbers DE-FG02-03ER46023 and DEFG02- 00ER15031) and from the NSF
FOCUS physics frontier centre. Use of the Advanced Photon Source
was supported by the U.S. Department of Energy, Office of Science,
Office of Basic Energy Sciences under Contract no. W-31-109-ENG-38).